1. Field of the Invention
The invention relates generally to electrolytic processes and apparatus for refining metals and, in particular, to an improved edge strip assembly for mother plates that have been modified by the removal of corner portions.
2. Description of the Prior Art
Electrolysis is utilized to extract metals and other cations from an electrolytic solution. The extraction process involves passing an electric current through an electrolyte solution of a metal of interest, such as copper, gold, silver, or lead. The metal is extracted by electrical deposition as a result of current flow between alternating anode and cathode plates immersed in cells of an extraction tank house. In electrowinning processes, a solution of metal-rich electrolyte is circulated through the extraction cells. The cathode is generally constructed of a metal alloy, such as titanium or copper alloys, and various grades of stainless steel which are resistant to corrosive acid solutions. Typically, each cathode consists of a thin sheet of metal of uniform thickness, e.g., 2-mm, disposed vertically between parallel sheets of anodic material, so that a uniform current density is maintained throughout the surface of the cathode. A pure layer of metal is electrodeposited on the cathode surface, which thus becomes plated during the process, upon passing of an electric current through the electrolyte.
Similarly, in refinery metal-purification processes, an anode of impure metal is placed in an electrolytic solution of the same metal and subjected to an electric current passing through the anode, the electrolyte and the cathode of each cell. The anode goes into solution, thereby separating the metal from impurities, which drop to the bottom of the tank. The electrical current then produces the deposition of the dissolved metal in pure form on the cathode, which typically consists of a mother plate of stainless steel. When a certain amount of pure metal has been plated onto the mother plate, the cathode is pulled out of the tank and stripped of the pure metal.
In both processes, the pure metal deposit is grown to a specific thickness in sheets deposited on each side of the cathode, and then the cathode is removed from the cell and stripped. For quality control purposes, it is very desirable that these sheet deposits be uniform in shape and thickness, so that they can be easily removed by automated stripping equipment. The overall economy of the production process depends in part on the ability to mechanically strip the cathodes of the metal sheets at high throughputs and speeds without utilizing manual or physical intervention. To that end, the mother plates have a surface finish that is resistant to the corrosive solution of the tank house and is strong enough to withstand continuous handling by automated machines without pitting or marking. Any degradation of the finish of the blank causes the electrodeposited metal to bond with the cathode resulting in difficulty of removal and/or contamination of the deposited metal.
If deposition is allowed to occur at the edges of a cathode, metallic bridges form between the deposited sheets on either side of the cathode. These metallic bridges, which can wrap around the edges of the cathode, hinder the stripping operation and can cause damage to the sheets and/or the cathode. To alleviate this problem, nonconductive strips known as edge strips or protector strips are placed over the submerged bottom and side edges of the cathode. The edge strips are normally mechanically fixed to the cathode with glued pins or pin inserts. In addition to inhibiting the formation of metallic bridges, edge strips function to prevent direct contact between the cathode and the adjacent anodes.
As is well understood in the art, at each lower corner of a cathode the vertical edge strips, mounted on the side edges of the mother plate, define junctions with the horizontal edge strip mounted on the bottom edge. Often glue is applied to these junctions to prevent penetration of the electrolyte into, and an accompanying deposition of metal around, the junctions. However, over time, the glue develops cracks which permit leakage of the electrolyte into the junctions. Eventually, the edge strips must be removed to allow removal of deposits accumulated on the edge of the cathode in the junction areas. The process of removing the edge strips, cleaning the cathode and replacing the edge strips is time-consuming and also keeps the cathode out of service; therefore, it is very undesirable.
In order to further reduce the likelihood of electrolyte penetration into corner junctions, manufacturers have improved the fit between abutting parts of adjoining strips. This objective has required greater precision finishing of the edge strips and more accurate positioning of the strips on the cathode during installation, so that manufacturing as well as mounting costs have increased. To obtain a better fit, edge strips are sometimes also mitered, which further increases manufacturing and installation time.
U.S. Pat. No. 5,690,798 describes a corner protector designed to wrap around the side and bottom edge strips abutting at the lower corners of a mother plate. The protector has a vertical channel adapted to receive the lower end of a vertical edge strip, a horizontal channel adapted to receive an end of the bottom edge strip, and a cutout for the corner of the mother plate. Thus, the protector provides additional separation between the edges of the cathode and the electrolytic solution, but it involves the use of an additional component with attendant supply, installation and maintenance costs.
A notable improvement recently found in the art, based on a different approach to reducing electrodeposition at the lower corners of cathodes, has been to cut away the corner portions of the mother plate covered by abutting side and bottom edge strips. Thus, the accumulation of electrolyte deposits is avoided by eliminating the metallic substrate upon which deposition may occur. Unfortunately, though, electrolyte seepage still causes deposition along the edges of the cut-away corners covered by the edge strips. The use of caulking and/or binding material, such as silicone, to seal the abutting parts of adjoining edge strips delays but does not prevent the eventual penetration of electrolyte and accumulation of deposits.
Therefore, there is still a need for an improved system of cathode-edge protection designed to overcome these problems, especially the accumulation in the corner areas of the mother plate. The present invention provides a new edge-strip component that fulfills this need for mother plates that have been modified by the corner cut-away approach described above.